VxPrint: add test mode

[adghayes]

The biggest question I have about test ballots on VxPrint is - are test ballots printed in the same time period as non-test ballots? In which case, you'd want to be able to print test ballots without clearing the printed ballots count. You could either:

• allow selecting "test" when printing from the print page
• don't clear printed ballot counts when toggling mode
  Otherwise, it would be our standard "clear all state" when switching mode.